The objectives of this research are (1) to isolate, purify and concentrate the antigenic components of Entamoeba histolytica by column chromatography, (2) to study the effect of high antibody titers on intracecal challenge with invasive E. histolytica, and (3) to determine if any of the antigenic components of E. histolytica might stimulate the production of protective antibodies in experimental animals. Guinea pigs will be immunized with sonicated antigen from Entamoeba histolytica and with three fractions of this antigen derived by column chromatography. The animals will be challenged intracecally with amebae. Seven days later the guinea pigs will be sacrificed and examined for gross lesions. Tissue will be taken for histolopathologic and for fluorescent antibody study, and cells will be collected for macrophage inhibition tests. Immunoelectrophoresis, indirect hemagglutination and 7-day passive cutaneous anaphylaxis will be performed with sera from these animals. Correlation between the presence of gross lesions and the levels of antibody and degree of hypersensitivity will be noted.